Field of the Invention
The present invention concerns a method for detection of a magnetic resonance (MR) signal in an examination region of an examination subject in a measurement field of an MR system with an MR imaging sequence, as well as an MR system for implementing such a method.
Description of the Prior Art
Known MR systems that use superconducting magnets to create the polarization field B0 typically have a spherical or cylindrical measurement field with a volume approximately 50 cm in diameter. In this measurement field, a high homogeneity of the polarization field is necessary for the creation of MR images. The magnet designs for generating the polarization field B0 are very complicated. For example, many coil elements and additional shielding coil elements are necessary so as to limit the polarization field to the inside of the MR system. Since superconducting magnets are cooled with helium, complicated devices are necessary to cool the many superconducting coils that are present. Each coil element has a number of windings, the position and clearance (separation) of which from the other windows are calculated depending on different criteria, for example with the goal of reducing the amount of superconducting wires. An additional cost-driving factor in MR systems is the consumption of helium for cooling the superconducting coils.
In spite of this, for MR measurements it is sometimes necessary to further improve the basic field homogeneity. For this purpose, shim coils are used that are supplied with a constant current at room temperature in order to further reduce the remaining B0 inhomogeneity. When a person to be examined is moved into the MR system, the currents through the shim coils can be defined as the static shims, which currents remain constant for the imaging. To further improve the homogeneity, it is possible to also adapt the shim currents before every MR imaging sequence. However, the currents used in the shim coils are constant over the duration of the imaging sequence. These shim coils are normally able to compensate for magnetic field inhomogeneities that are on the order of 10 ppm or less. This correction takes place primarily in order to correct B0 inhomogeneities that result due to magnetic susceptibility effects when the examined person is moved into the MR system.
In addition to the high costs for the MR system itself, the opening for introducing the person to be examined should be as small as possible, so as to provide a homogenous polarization field over the measurement field. Due to the restricted space available in the MR magnet, the examination is uncomfortable for many people; examinations of patients with claustrophobia may even be declined due to the restricted available space.